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We present long-slit optical spectra along the major axis of the planetarynebula NGC 7009. These data allow us to discuss the physical, excitation andchemical properties of all the morphological components of the nebula,including its remarkable systems of knots and jets. The main results of thisanalysis are the following: i) the electron temperature throughout the nebulais remarkably constant, T_e[OIII] = 10200K; ii) the bright inner rim and innerpair of knots have similar densities of N_e = 6000cm^{-3}, whereas a much lowerdensity of N_e = 1500cm^{-3} is derived for the outer knots as well as for thejets; iii) all the regions (rim, inner knots, jets and outer knots) are mainlyradiatively excited; and iv) there are no clear abundance changes across thenebula for He, O, Ne, or S. There is a marginal evidence for an overabundanceof nitrogen in the outer knots (ansae), but the inner ones (caps) and the rimhave similar N/H values that are at variance with previous results. Our dataare compared to the predictions of theoretical models, from which we concludethat the knots at the head of the jets are not matter accumulated during thejet expansion through the circumstellar medium, neither can their origin beexplained by the proposed HD or MHD interacting-wind models for the formationof jets/ansae, since the densities as well as the main excitation mechanisms ofthe knots, disagree with model predictions.

The Physical Parameters, Excitation, and Chemistry of the Rim, Jets, and Knots of the Planetary Nebula NGC 7009